1. Field of the Invention
The present invention relates to a method of reducing frictional resistance between a ship body and water by releasing gases in water, and more particularly to a method of releasing gases at a predetermined position under the water level of a fore part of the ship body according to parameters such as the total length, velocity and draft depth of a ship to provide advantageous interferences to lower the average density ρ of water at the water surface in contact with the ship body, and the released gases at a high pressure zone can reduce the pressure of water to a hull and the suction of water to the hull in a low pressure zone to provide both functions simultaneously.
2. Description of the Related Art
In general, a vessel refers to a motive power ship, a warship, an aircraft carrier, a liner, a cargo ship, an oil tanker, a yacht, a waterjet, a model ship, a seaplane, a no-power oil drilling platform, and a barge, etc.
Different vessels come with different functions and body designs. For example, warships require a high velocity, and thus the warships are usually designed in a slender streamlined shape to reduce the carrying capacity and draft relatively. Merchant ships require a large carrying capacity and a high draft, and thus the merchant ships are usually designed in a broad shape, but such shape will reduce the velocity, since the resistance of water such as seawater and fresh water (both referring to “water”) hinders the ships and retards the velocity.
Referring to FIGS. 1 to 3 for a perspective view, a side view and a top view of a liner 20 respectively, the commercial liner 20 generally comes with a length L of hundreds of meters (m), a beam width B of 60˜70 m, and a draft depth D of 15˜25 m depending on the load of the liner 20. Therefore, the large-sized merchant ships of this short have a large contact area with water. According to the principle of Fluid Mechanics, the friction drag FD can be calculated by the following equation:
      F    D    =            C      D        ⁢    A    ⁢                  ⁢          1      2        ⁢    ρ    ⁢                  ⁢          U      2      
wherein CD is the coefficient of drag;
A is the total area in contact with the fluid;
ρ is the fluid density;
U is the velocity (m/s).
For example, a hull 30 of the liner 20 of this embodiment is simulated as a slab in contact with seawater, and the slab comes with a length L of 360 m, a beam width B of 70 m, and a draft depth of D 25. If the liner 20 is traveling at a velocity of 13 knots in seawater at a temperature of 10° C., the Equation
  FD  =            CDA              1        2              ⁢    ρ    ⁢                  ⁢    U    ⁢                  ⁢    2  can be used to calculate the friction drag at the surface of the hull 30. As described in a book “Introduction to Fluid Mechanics”, Sixth Edition, authored by Professor Robert W. FOX, et al of Purdue University, the values of the abovementioned L, B, D and U can be used for calculating the value of FD, if the velocity is 13 knots (wherein a knot is equal to one nautical mile per hour). Therefore,Velocity U=13 nm/hr×6076 ft/nm×0.305 m/ft×hr/3600 s=6.69 m/s
Further, the kinematic viscosity v=1.37×10−6 m2/s at a temperature of 10° C., and such kinematic viscosity and the data mentioned in Professor Fox's book can be used to obtain CD=0.00147 and ρ=1020 kg/m3.
The hull 30 is in contact with an area having a length and a width such that W=B+2D of the seawater, and thus the total contact area A with the seawater=360 m×(70+50) m=43200 m2.
From the equation above,
                                          F            D                    =                    ⁢                                    C              D                        ⁢            A            ⁢                                                  ⁢                          1              2                        ⁢            ρ            ⁢                                                  ⁢                          U              2                                                                    =                    ⁢                      0.00147            ×            43200            ⁢                                                  ⁢                          m              2                        ×                          1              2                        ×            1020            ⁢                                                  ⁢                          kg                        ⁢                          /                        ⁢                          m              3                        ×                                                                  ⁢                                                    (                6.69                )                            2                        ⁢                                                  ⁢                          m              2                        ⁢                          /                        ⁢                          s              2                        ×                          N              ·                              s                2                                      ⁢                          /                        ⁢                                          kg                            ·              m                                                FD    =          1.45      ⁢                          ⁢      MN      
The relative power isP=FDU=1.45×106 N×6.69 m/s×W·s/N·m
P=9.70 MW which is approximately equal to 13000 HP, and this data shows that the power required for overcoming the friction drag at the surface of the liner 20 is very large.
From the example above, we know that the resistance comes from the air, tire, and road surface when the ship is moving on land, and the resistance comes from the air resistance and the friction drag between the hull and water when the ship is moving in sea. With the same motive force, the speed of the ship traveling on land is faster than that traveling in sea, since the density of water is 1 at 20° C. and 1 atmospheric pressure, and the density of water is approximately 800 times of the density of air.
Therefore, manufacturers and designers in the related fields conduct extensive researches and attempt to reduce the friction drag FD between the hull and water. In some of the conventional methods, the reduction the friction drag between a hull and water is generally limited to the installation of a bulb bow and a foil, wherein the bulb bow is provided for reducing the resistance caused by a wave breaking and a reflection at the fore part, and the foil is provided for supporting and slightly lifting the ship body to reduce the water contact area with the ship body. However, such arrangement can be applied to small-sized vessels only. For example, a hovercraft requires a large quantity of air to support the hovercraft body, and thus the aforementioned arrangement can be applied to small-sized vessels only, since the density ρ of water cannot be decreased, and the friction drag between the hull and water, the pressure in a high pressure zone and the suction in a low pressure zone still exist.
In FIG. 3, a thin boundary layer is disposed between a range (a→b) from a fore part 31 of the hull 30 to a point c at the after part through the ship body and water. As far as there is a friction at the boundary layer, the drag still exists, such that an object will produce a track 32 of wakes as indicated by the line before the Point c in FIG. 3. Point c is a separation point, where fluid particles are separated from the object to produce a track, and form a wake 33 on the internal side of the Point c, which is the flow line from Point a to Point b and considered as a high pressure zone. Turbulence or a low pressure zone is formed in a small area behind the Point b, and the wake 33 behind the Point c forms a low pressure zone. Regardless of the pressure produced by water currents to the hull 30 in the high pressure zone and the suction produced by water currents to the hull 30 in the low pressure zone, both pressure and suction constitute a resistance to a forward movement of the ship, and thus it is necessary to overcome the pressure and suction to enhance the efficiency of the ship.
Another conventional way of reducing the friction drag applies a new technology to design the shape of the ship by computers and hydraulic experiments, so that interferences are produced to water waves, but such arrangement is effective in a range of velocities only, but the shape of the ship cannot changed to provide favorable interferences to the water waves according to different velocities, and thus the effect achieved by this conventional method is limited.